mcode force allocation on Android is now optional

Anyone who encounters random freeze of koreader for Android
is encouraged to enable this by adding a file at
"/sdcard/koreader/patch.lua" with the content of:
```
require("jit.opt").start("sizemcode=256","maxmcode=256")
for i=1,100 do end  -- Force allocation of one large segment
```
The sizemcode and maxmcode could be adjusted from 196 to 512 to
make koreader best fit on your Android device.

platform/android/llapp_main.lua

@@ -23,6 +23,9 @@ local file = A.jni:context(A.app.activity.vm, function(JNI)
 end)
 A.LOGI("intent file path " .. (file or ""))
 
+-- run koreader patch before koreader startup
+pcall(function() dofile("/sdcard/koreader/patch.lua") end)
+
 -- create fake command-line arguments
 arg = {"-d", file or "/sdcard"}
 dofile(A.dir.."/reader.lua")